About 280 million years ago, all
the continents were assembled together into a supercontinent--Pangea.
Present day “continental" Oregon
did not exist. About 170 million years ago, Pangea broke apart
along boundaries resembling present day continental margins. The
break-up started with a spreading rift system (red dashed line)
forcing North America to move northwest and creating the Atlantic
Ocean as it widened. The splitting-up of Pangea motion initiated
the coastal subduction zone that strongly influences Oregon geology.
The blue dotted line shows the ancient shoreline of western North
America at this time.